Consider a system of linear equations like 1, where the number of rows equals the number of
unknowns, i.e. *p*=*n*. We can write it in matricial form *AX*=*B*, where

*A*=*a*_{ij}is the coefficient matrix of the system (v.s. refrestricted matrix)*B*is a matrix of order , whose single column is identical to the last column of the augmented matrix of the system (v.s. refaugmented matrix).*X*is the unknown one-column matrix .

Suppose that *A* is invertible. We have: